AQA AS Level Biology - simplified cards on the uptake and adaptations of xerophytes

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Uptake of water in flowering plants

There are 2 ways that water can move through plant roots:

  • APOPLAST PATHWAY - through this ppathways there are no barriers to movement so H2O is able to diffuse freely. This route doesn't involve H2O entering cells. It passes through cell walls. In order for this pathway to work there must be a water potential gradient down which H2O can diffuse. Water is drawn through this pathway duering transpiration
  • SYMPLASTIC PATHWAY - depends on the water potential gradient. H2O moves through cytoplasm from one cell to another as the cells in the cortex aren't completely serparate. Channels in cell walls mean that the cytoplasm of one cell continues into the cytoplasm of the next cell. These channels are called plasmodesmata.
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Plant uptake of water

There are 3 ways that plants take H2O from the roots up the xylem. It is though all 3 ways are used by plants to take up H2O:

1. COHESION-TENSION THEORY - transpiration causes evaporation of H2O from the leaves of the plant. This leads to a low pressure and causes tension in the column of H2O inside xylem more H2O is pulled up the stem to replace evaporated H2O. Hydrogen bonds between the H2O molecules allow them to stick together, know as cohension. This means that when one molecule of water is pulled up, more H2O  in xylem is moved up by mass flow

2. ROOT PRESSURE - Ions are pushed into xylem from the endodermis by active transport. This causes a bigger water potential gradient to form. H2O moves into xylem which pushes up the H2O thats already there. This pressure is weak and couldn't more H2O on its own. But it helps young plants to have few leaves to lose enough H2O by transpiration for cohesion-tension to occur

3. CAPILLIARITY - this is when H2O moves up the thin tubes of the xylem. The movement is due to the attraction between the water molecules and the inside of the xylem tuve

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Adaptations of xerophytes

XEROPLANTS - plants that live adapted to very dry environment. Adaptations:

1) Stomata - many have fewer which are sunken decreasing exposure to air currents so less H2O is lost by transpiration. Some xerophytes only open their stomata at night when its dark and cooler to limit H2O loss by transpiration

2) Leaves - some have small leaves to reduce the surface area which will reduce H2O loss by transpiration. Others have vertically orientated leaves to decrease the surface area exposure to light and heat and decrease H2O lost. Other have roled leaves to prevent H2O loss and maintain humid air around the stomata. This prevents exposure to air current and reduces the water potential gradient. They also reduce surface area of the leaf that is exposed to heat, light and waind which will reduce transpiration further.

3) Cuticle - many xerophytes have thick waxy cuticles again to prevent H2O loss

4) Stem - some have hairs convering stem. They help shade and cool the plant leading to a reduced rate of transpiration. They also trap a layer of moist air which decreases exposure to air currents and reduces the concentration gradient of water and air between the plant and the atmopshere. So plant loses less H2O

5) Roots - xerophytes have a large and widespread root system system so they can take in as much H2O as possible. The cortex of their roots there is a shorter distance between H2O in the soil and the xylem. So H2O can be taken up quicker

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